Potiential nullptr derefence in io_do_iopoll

[chase xd]

Dear maintainers,

I'm looking into io_uring internals and find there might be a bug in
io_do_iopoll, but I'm struggling to construct a POC due to my lack of
knowledge about the kernel internals. So maybe it's better to put it
here for discussion.

After [issuing an
SQE](https://elixir.bootlin.com/linux/v6.9/source/io_uring/io_uring.c#L1920)
in iopoll-enabled io_uring, if the return value is
`IOU_ISSUE_SKIP_COMPLETE` and the op supports `iopoll_queue`, the req
will be added to `ctx->iopoll_list` and later retrieved in
`io_do_iopoll`, where `iopoll` or `uring_cmd_iopoll` of the req file
op is [called](https://elixir.bootlin.com/linux/v6.9/source/io_uring/rw.c#L1167).

IMHO here we miss a check of whether `iopoll` or `uring_cmd_iopoll` is
implemented. A more understandable case for me is, the custom ioctl
function with [IORING_OP_URING_CMD](https://elixir.bootlin.com/linux/v6.11.7/source/io_uring/opdef.c#L416)
satisfies all the constraints and will go to this path if `uring_cmd`
returns `-EIOCBQUEUED`
[here](https://elixir.bootlin.com/linux/v6.9/source/io_uring/uring_cmd.c#L192).

So this requires that all the ops with `->uring_cmd` returning
`-EIOCBQUEUED` should support `->uring_cmd_iopoll` as well, which is
not the case for
[ublk_ch_fops](https://elixir.bootlin.com/linux/v6.9/source/drivers/block/ublk_drv.c#L1967)
resulting in a nullptr-deref in `io_do_poll`. I'm wondering if this is
legit.

Note that the related code changes a bit for [newer
kernel](https://elixir.bootlin.com/linux/v6.11/source/io_uring/uring_cmd.c#L261).

Best Regards